He cheated on me. The word has this resonance of betrayal, of lies and of heartbreak. We imagine furtive texts, secretive rendezvous, hot sex and mixed emotions. And the culture reinforces this: in case after case, months, years, or decades of a relationship, of a marriage, and potentially of a family are all routinely thrown out the window because of one night of steamy infidelity.

We're taught the other side as well,that we all secretly want to cheat and that the moral thing to do is to control ourselves. There's this human nature argument there at the root of it, we're just primates with these animal urges to fuck everything we can, and what makes us human is our sense of restraint.

And what if only half that sentence were true? What if, at the root if it, we are just primates? And what if the type of relationship we've been told to have for our entire lives ends up doing us and our partners more harm than good?

Many point to monogamy among humans developing around the time of the agricultural revolution, when humans finally figured out how to farm and started setting up shop, some 12,000 years ago. This is the traditional story taught in most high school history classes: as we as a species began to settle down, men and women fell into the gender roles we still see today, with men and women trading food, shelter and
protection for sexual fidelity and reproduction.

This trade-off, argues Anthropologist Christopher Ryan, in a TED talk on the human roots of non-monogamy, has pitted men and women against each other in a sexual dynamic that would haunt us for thousands of years.

Christopher Ryan during the Ted Talk[/caption]

Still, monogamy has only been common among humans for a very small part of our history. Ryan explains that before settling down as a species, we lived in hunter-gatherer groups that were based around a fairly radical sense of egalitarianism. Food, shelter and protection were all shared among the group, he says, and sexuality served as much of a way of keeping the group together as a way of procreation.

We're not alone in that idea. Bonobos, our ape cousins, are also notoriously polyamorous, Ryan says, and like pre-agricultural humans, hold sex as a way to create and maintain bonds among the group. As far as reproduction and evolution are concerned, the more sperm that compete for an egg, the better. That's why female bonobos and chimpanzees will mate with over ten males per day when they're looking to reproduce.

Twelve thousand years later, we've got a situation where monogamy is the norm and where many studies show that about half of men and women in relationships have cheated on their partners.

So why do we cheat?

Monogamy assumes that two people are taking full responsibility for each other's sexuality. So if one half of a couple is into getting tied up, or sleeping with people of other genders, or threesomes, or whatever else, and the other isn't, you've got a big problem.

And if the idea of fucking the same person for the rest of your life is slightly terrifying, you're not alone.

“There are people out there who aren't any good at monogamy,” writes sex columnist Dan Savage, “Some people are incapable of fucking just one person for four, five, or six decades.”

Savage has long argued, not that non-monogamy is for everybody, but that the honest conversation over how to run your relationship absolutely is. We should make the rules for our own relationships, Savage says, and not rely on the model of what we're socially expected to do.

“We’re advocating for honesty in our sexual relationships—be they open or closed—and an informed, realistic approach to whatever arrangements we make with our partners,” Christopher Ryan, the bonobo anthropologist, writes as a guest on Savages column.

Besides, what exactly constitutes cheating, the act or the intent? Is it cheating to fantasize about another person? What about flirting, cuddling, kissing or sex with them? Isn't spending a decade fantasizing about somebody else while having sex with your partner “worse” than actually having one hot night of infidelity with that person?

There is no right answer here: It's different for everybody, and there is no one arrangement that will work for every couple. Even if we do decide on monogamy, we've got a lot to gain from having an honest conversation about sex and sexuality.

Ryan puts it differently. Why does he advocate for stepping out of the social mold of monogamous relationships?

“Because informed, realistic, bullshit-free relationships are richer, more intimate, sexier, and ultimately far more durable than relationships built on Disney-fed fantasies of virginal princesses, eternal bodice-ripping passion, and eyes that never wander.”